The present invention relates to a method for preparing hydrides containing at least two different atoms from Group 4A of the Periodic Table wherein at least one of the Group 4A atoms is silicon or germanium. Such hydrides can be used as deposition feedstock materials for the formation of hydrogenated amorphous silicon alloy films in the fabrication of photovoltaic devices and other semiconductor devices.
A number of different ways for making silicon and germanium hydrides containing two different Group 4A atoms have been reported in the literature. In one method, mixtures of different Group 4A hydrides are fed into a silent electronic discharge, sometimes referred to as an ozonizer-typ electric discharge. See T. D. Andrews and C. S. G. Phillips, "Further Studies on the Silicon-Germanium Hydrides," J. CHEM. SOC. (A) (1966) pp. 46-48; S. D. Gokhale, J. E. Drake and W. L. Jolly, "Synthesis of the Higher Silanes and Germanes," J. INORG. NUCL. CHEM. (1965) Vol. 27, pp. 1911-1916; E. J. Spanier and A. G. MacDiarmid, "The Synthesis of Germylsilane from Silane and Germane in a Silent Electric Discharge," INORG. CHEM. (1963) Vol. 2, pp. 215-216.
In another method, alloys of magnesium and different Group 4A metals are reacted with acid. See P. L. Timms, C. C. Simpson and C. S. G. Phillips, "The Silicon-Germanium Hydrides," J. CHEM. SOC. (1964) pp. 1467-1475 (Timms et al.); C. S. G. Phillips and P. L. Timms, "Some Applications of Gas Chromatography in Inorganic Chemistry", J. ANAL. CHEM. (April 1963) Vol. 35, No. 4 pp. 505-510. Other methods for preparing these materials include: pyrolysis of a germane with a silane; and the reaction of hydrofluoric acid on a mixed SiO-GeO preparation. See Timms et al.
A problem with these processes, however, is that they are not selective. That is, a number of different hydrides containing Group 4A atoms are formed. Thus, extensive purification is required to make a particular hydride by these processes. A more selective method for preparing these hydrides is therefore desirable.
One possible more selective route for preparing silicon and germanium hydrides containing two different Group 4A atoms that has been tried is the reaction of potassium silyl (KSiH.sub.3) with a halide such as CH.sub.2 Cl.sub.2, CHCl.sub.3, or CH.sub.2 Br.sub.2. J. A. Morrison and J. M. Bellama, "Synthesis and Characterization of the (Halosilyl) Methyl Silanes," JOURNAL OF ORGANOMETALLIC CHEMISTRY (1975) Vol. 92, pp. 167, report that this process route was found suitable for small-scale preparations of (SiH.sub.3).sub.2 CH.sub.2. However, attempts to prepare this hydride on a larger scale at room temperature without solvent usually resulted in detonation.
Another problem with this process route is the difficulty and length of time it takes to prepare the starting material, potassium silyl (KSiH.sub.3). M. A. Ring and D. M. Ritter "Preparation and Reactions of Potassium Silyl," J. AM. CHEM. SOC., (1960) Vol. 83, pp. 802 report that their preparations of potassium silyl (KSiH.sub.3) using potassium metal and monosilane (SiH4) took 60, 70, 74, and 76 days respectively. Ring and Ritter also report therein the preparation of sodium-potassium silyl from an alloy of sodium and potassium and monosilane (SiH.sub.4) in 14 days. Starting with disilane (Si.sub.2 H.sub.6), R. Varma and A. P. Cox, "A New Synthesis of Germylsilane," ANGEW. CHEM. Internat. Edit. (1964) Vol. 3, No. 8, p. 586, report the preparation of potassium silyl (KSiH.sub.3) over a 24 hour period.